The side portion of a persons leg that is exposed while seated on the toilet. This includes (but is not limited to) the lower calf, the thigh and the outer butt cheek. Named after its resemblence to the rear side portion of the equine species. Often the first sight of someone who opens an occupied bathroom door that should have been locked.

(roomate opens bathroom door, only to find his housemate having a bowel movement)
-"DUDE! Lock the door when you're taking a dump. No one wants to see your horse portion!"